The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for performing chemical species separation, such as water-fat separation, using an MRI system.
Chemical shift encoded techniques for water-fat separation have experienced considerable development and application in recent decades. Originally proposed by W. T. Dixon in “Simple proton spectroscopic imaging,” Radiology, 1984; 153(1):189-194, and subsequently expanded by G. Glover and E. Schneider in “Three-point Dixon technique for true water/fat decomposition with B0 inhomogeneity correction,” Magnetic Resonance in Medicine, 1991; 18(2):371-383, these techniques have been adopted by applications that require improved visualization of water-based tissues as well as those that demand robust fat suppression in areas of severe B0 field inhomogeneity. In addition, the use of chemical shift encoding in T1-weighted contrast enhanced imaging is particularly important because short T1 inversion recovery (“STIR”) techniques are incompatible with post-contrast T1-weighted imaging. A variety of water-fat separation techniques have been proposed, including a single-echo method, dual-echo methods, and numerous methods that utilize three or more echoes.
A significant challenge in complex-based water-fat separation is accurate estimation of the B0 field because the least-squares cost is a non-linear and non-convex function of the B0 field map. If the B0 field map is estimated accurately, then the water and fat signals can be separated using a straightforward linear inversion. However, inaccurate estimation of the B0 field map can lead to “swaps” of the water and fat signals. This is a commonly recognized challenge for chemical shift encoded water-fat separation methods.
Accurately estimating the B0 field map has been a major focus of technical development in water-fat separation. Many techniques assume that the B0 field is slowly varying. Although this assumption is empirically based, it is sufficiently valid in many cases, which explains the effectiveness of these techniques. However, none of these methods use any anatomical information or other geometrically based information to aid in the determination of the B0 field map. Recent work from H. Yu, et al., described in “Robust multipoint water-fat separation using fat likelihood analysis,” Magnetic Resonance in Medicine, 2012; 67(4):1065-1076 has exploited the material properties of tissue by exploiting spectral complexity of fat to minimize water-fat swaps.
Despite the relatively successful performance of most techniques, water-fat swaps still occur. This is especially true in spatial regions where the B0 field varies rapidly because the common assumption of a slowly varying B0 field becomes invalid. Furthermore, images with discontinuous regions of tissue separated by air or low signal are also prone to water-fat swapping since region growing methods aimed at estimating the B0 field map are unable to interpolate field map estimates accurately across regions of noise.
It would therefore be desirable to provide systems and methods for producing a more robust estimate of the B0 field map that overcomes the drawbacks mentioned above, such that accuracy of chemical species separation and other imaging methods that make use of a B0 field map estimate can be improved.